1. Technical Field
The present invention relates to a container utilized, for example, for containing, storing, and transporting a great number of thin plates such as semiconductor wafers or liquid-crystal glass plates, and more particularly to the structure of a gasket that enables the inner pressure of the container to accommodate fluctuation in pressure conditions. PCT Patent Publication WO 2004/035423 entitled “Storage Container for Receiving Precision Substrates such as Wafers” is incorporated into this specification by reference.
2. Description of the Related Art
Because ultramicroscopic processing is to be applied to the surfaces of substrates (thin plates), such as semiconductor wafers, that require precision packaging, extremely high-level cleanliness is required to store, transport, and contain the substrates; therefore, extremely airtight substrate containers are utilized. However, when, for example, an extremely airtight container is carried by airplane, as the altitude increases, a difference between the atmospheric pressure and the inner container air pressure occurs, whereby the container expands and the inner container gas is released for awhile; then, on the ground, the inner container air pressure is low compared with the atmospheric pressure, causing the container to contract. With the container contracted, it is very difficult to open the lid; if the lid is forced open, the atmospheric air abruptly enters the container and a drawback occurs, causing dust in the atmospheric air to contaminate the substrates.
A wafer container is disclosed that employs a configuration in which, in order to address the difference between the atmospheric pressure and the inner container air pressure, a molecular separation filter is mounted in a port provided in the sidewall of the container body or the lid (Japanese Utility Model Application No. 1990-49721 (FIG. 1)). It is argued that, with the wafer container, because the air is discharged from or flows into the container through the molecular separation filter in response to the fluctuation in the atmospheric pressure, opening and closing the lid is readily carried out. However, measures using a shielding plate M that is annexed opposite the inner side of the molecular separation filter F are also disclosed, in order to address a problem that, under the atmospheric pressure fluctuation that occurs while the container is transported as airborne freight, the air is discharged from or flows into the container through the molecular separation filter, causing dust in the atmospheric air (the air within the cargo compartment of the airplane) to adhere to the filter and causing particles that have passed through the filter to adhere to the surface of substrates contained on the opposite side of the filter (Japanese Laid-Open Patent Publication No. 1998-107135 (FIG. 3)). In addition to this problem, the fact that the molecular separation filter requires cleaning prior to each air flight has posed a problem in terms of economic efficiency and usefulness.
Meanwhile, a structure is disclosed in which a gasket is formed of a fluorine-system resin or an olefin-system resin that is an open-cell foamed material made up of continuous pores, and that is mounted interposed between a container body for precision packaging of substrates, such as semiconductor wafers, and the lid (Japanese Patent Application Laid-Open No. 2003-170969, paragraph 0021). It is argued that the foregoing gasket facilitates ventilation through the pores and the inner container air pressure can always be maintained at the same level as the atmospheric pressure, regardless of change in the atmospheric pressure.
The gasket made up of the open-cell foamed material is adapted to make the pores trap dust in the atmospheric air; therefore, small-diameter dust passes through the pores, but large-diameter dust is trapped in the pores. Accordingly, the formation of the gasket requires strict management of the pore diameter and adjustment and has a limited number of uses, thereby being expensive.